Gain equalization system and method

ABSTRACT

A power equalization system and method for use in an optical transmission system are provided. The power equalization system includes an optical line including at least one transmission channel and a management line. The transmission system further includes a plurality of amplifiers, a plurality of Optical spectrum analyzers and a plurality of equalizers. The plurality of amplifiers are coupled to the optical line, spaced periodically throughout the optical transmission system. As information is sent through the optical transmission system, the plurality of amplifiers boost the power of each channel of the optical signal. A plurality of optical spectrum analyzers are also coupled to the optical lien and are spaced periodically throughout the optical transmission system and are co-located with a first portion of the amplifiers coupled to the optical line. A plurality of equalizers are also coupled to the optical line and are spaced periodically throughout the optical transmission system and equalize the power on each channel of the optical line. The plurality of equalizers are co-located with a second portion of the plurality of amplifiers and at least one of the plurality of Optical spectrum analyzers is not co-located with one of the plurality of equalizers. As optical information is transmitted over the optical transmission system, the Optical spectrum analyzers provide analysis data via the management line to the non co-located equalizers for use by the equalizers in equalizing the power of the channels of the optical line at that point. The analysis data generated by the Optical spectrum analyzer identifies the analysis data at the point of the Optical spectrum analyzer which is not co-located with the equalizer.

CROSS--REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

Not Applicable.

FIELD OF THE INVENTION

This invention relates to an equalization system, and more particularly,to a system for use in an optical network to correct for unequal gain ofpower in the channels of the optical signal.

BACKGROUND OF THE INVENTION

The transmission, routing and dissemination of information has occurredover computer networks for many years via standard electroniccommunication lines. These communication lines are effective, but placelimits on the amount of information being transmitted and the speed ofthe transmission. With the advent of light-wave technology, a largeamount of information is capable of being transmitted, routed anddisseminated across great distances at a high transmission rate overfiber optic communication lines.

When information is transmitted over fiber optic communication lines,impairments of the pulse of light carrying the information can occur,including pulse broadening (dispersion) and attenuation (energy loss).In-line amplifiers spaced throughout the fiber optic communicationsystem boosts the power of each channel of the optical signal to assistin the compensation of the energy lost during transmission. The in-lineamplifiers boost each channel of the optical signal with the same amountof power. However, as different wave lengths of light are used over thedifferent channels of the fiber optic communication system, the amountof energy lost per channel is not consistent. As the in-line amplifierboosts the energy across all channels of the optical signal transmittedover the fiber optic communication system, the power gain of anyspecific channel may fail to meet or exceed the desired power gain.Further, energy loss caused by polarization dependent loss (PDL) lead tofurther nonuniform power gain over the multiple channels of the opticalsignal transmitted over the fiber optic communication system.

As the optical signal is transmitted across the fiber opticcommunication system, the gain differences on a channel-by-channel basisaccumulate. These gain differences can cause distortions of the opticalsignal shape and therefore lead to performance degradation. Currentsystems allow for the optical signals' power deviations to accumulatebefore they are compensated by the gain equalizer after analysis by theoptical spectrum analyzer (“OSA”). Inherent in these systems is aprocess which allows a large amount of gain differences to accumulateprior to equalization. Prior to the gain equalization of the channels ofthe optical signal, the optical signal performance beings to degrade andthus the overall performance of the fiber optic communications system isdegraded.

To compensate for gain differences in the multiple channels of theoptical signal, gain equalizers are provided, spaced periodically,throughout the fiber optic communication lines (FIG. 1). the gainequalizers equalize the power at the in-line amplifiers on achannel-by-channel basis throughout the optical signal. To determine theamount of gain on a channel-b-channel basis, an optical spectrumanalyzer is co-located with the gain equalizer. The Optical measures thepower level associated with each channel of the optical signal andcompares this power level with the desired power level for each channeland provides this information to the gain equalizer which is co-locatedwith the Optical spectrum analyzer at an in-line amplifier within thefiber optic communication system. The gain equalizer then equalizes thepower of each channel based upon the analysis performed by the Opticalspectrum analyzer at this in-line amplifier location. As can be seen inFIG. 2, the gain equalizer zeroes out the gain difference throughout thechannels at the point in the fiber optic communication system where thegain equalizer and Optical spectrum analyzer are located. Therefore, anyadvancement in the ability to lower the amount of gain differenceaccumulated during the transmission of information over a fiber opticcommunication system would be advantageous.

SUMMARY OF THE INVENTION

A power equalization system and method for use in an opticaltransmission system are provided. The power equalization system includesan optical line including at least one transmission channel and amanagement line. The transmission system further includes a plurality ofamplifiers, a plurality of Optical spectrum analyzers and a plurality ofequalizers. The plurality of amplifiers are coupled to the optical line,spaced periodically throughout the optical transmission system. Asinformation is sent through the optical transmission system, theplurality of amplifiers boost the power of each channel of the opticalsignal. A plurality of Optical spectrum analyzers are also coupled tothe optical lien and are spaced periodically throughout the opticaltransmission system and are co-located with a first portion of theamplifiers coupled to the optical line. A plurality of equalizers arealso coupled to the optical line and are spaced periodically throughoutthe optical transmission system and equalize the power on each channelof the optical line. The plurality of equalizers are co-located with asecond portion of the plurality of amplifiers and at least one of theplurality of Optical spectrum analyzers is not co-located with one ofthe plurality of equalizers. Thus, as the optical information istransmitted over the optical transmission system, the Optical spectrumanalyzers provide analysis data via the management line to the nonco-located equalizers for use by the equalizers in equalizing the powerof the channels of the optical line at that point. The analysis datagenerated by the Optical spectrum analyzer identifies the analysis dataat the point of the Optical spectrum analyzer which is not co-locatedwith the equalizer.

DETAILED DESCRIPTION OF THE DRAWINGS

A better understanding of the invention can be obtained from thefollowing detailed description of one exemplary embodiments asconsidered in conjunction with the following drawings in which:

FIG. 1 is a block diagram depicting an optical transmission systemaccording to the prior art;

FIG. 2 is a graphical representation of the accumulated gain of anoptical signal being transmitted over an optical transmission systemaccording to the prior art;

FIG. 3 is a graphical representation of the loss per kilometer ofdifferent wavelengths of an optical signal transmitted through anoptical transmission system;

FIG. 4 is a block diagram depicting the optical transmission systemaccording to the present invention;

FIG. 5 is a graphical representation of the optical spectrum analysis ofthe gain per channel of an optical signal transmitted over the opticaltransmission system according to the present invention;

FIG. 6 is a graphical representation of the accumulated gain per channelof an optical signal transmission across the optical transmission systemaccording to the present invention; and

FIG. 7 is a graphical representation of the accumulated gain per channelof the optical signal after the gain equalization has occurred accordingto the optical transmission system according to the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In the descriptions which follows, like parts are marked throughout thespecification and drawings with the same numerals, respectively. Thedrawing figures are not necessarily drawn to scale and certain figuresmay be shown in exaggerated or generalized form in the interest ofclarity and conciseness.

In an optical transmission sytem, the optical signal is transmitted overan optical line. In certain optical transmission systems, includingdense wavelength division multiplexed systems (DWDM systems), theoptical signal that is transmitted includes multiple channels where eachchannel of the optical signal is transmitted at a different wavelength.As originally transmitted, the optical signal does not contain enoughenergy to complete a long haul or ultra long haul transmissiontherefore, the optical signal must be amplified periodically throughoutthe optical line. The amplifiers are used to replace energy loss due toattenuation with an equal amount of replacement energy per wavelength(or per channel) in the optical signal. There are multiple wavelengthsor channels in each optical signal transmitted over the opticaltransmission system. In one disclosed embodiment, forty wavelengths arecapable of being transmitted through one optical signal. As the amountof power attenuation varies among the channels of the optical signal,some channels experience an excessive amount of gain caused by theamplifier, some experience the correct amount of gain and someexperience too little gain. These gain differences experienced bydifferent channels of the optical signal are accumulated and are causedby the wavelength profile of the amplifiers or by effects such aspolarization dependent loss (PDL) in the various components located inthe optical system. The losses per wavelength are not perfectly flat.The number of energy lost between any two points along the opticalnetwork will typically remain nearly constant if the lost power iscaused by the amplification of the amplifiers. A random gain deviationcan be experienced intermittently and is typically not consistent.

In a conventional optical networks 100, as can be seen in FIG. 1, gaindeviation is corrected through the use of optical spectrum analyzers 108and dynamic gain equalizers 110. The conventional optical network 100includes terminals 102 and 104. The terminals 102 and 104 connect theremaining conventional optical network 100, which is typically a densewavelength division multiplexed (DWDM) network to the local opticalnetwork (not shown). The terminal 102 receives an optical signal fromthe local optical network and transmits this optical signal across theconventional optical network 100 to terminal 104. For the sake ofillustration, the conventional optical network 100 is shown operating inonly one direction; however, it is well known to those skilled in theart that an optical network can function bi-directionally.

The optical signal 101 is typically comprised of twenty to fortychannels. Each channel of the optical signal 101 is a separatewavelength of the optical signal. The optical signal 101 is typicallytransmitted in wavelengths between 1530 and 1610 nm. As the opticalsignal 101 is transmitted from the terminal 102 over the optical network100, the optical signal 101 begins to experience attenuation or powerloss. therefore, an in-line amplifier 106 is provided at fixed intervalsto boost the power of the optical signal 101.

The in-line amplifier 106 boosts the power of the optical signal 101according to a predetermined value or according to a value that can beadjusted during operation. The In-line amplifier 106 boosts the power ofthe optical signal 101 across every channel, applying the same powerboost to every channel regardless of the channels current power level.Therefore, as gain differences are experienced in the multiple channelsof the optical signal 101, these gain differences are accumulatedthrough the amplification of the optical signal 101 from the In-lineamplifiers 106. As the optical signal 101 is transmitted from In-lineamplifier 106a to In-line amplifier 106b, the optical signal 101experiences attenuation and again must be amplified.

However, prior to amplification at In-line amplifier 106b, an opticalspectrum analyzer 108a evaluates each channel of the optical signal 101to determine its current power deviation from the expected value. Theoptical signal analyzer 108a then transmits this power or gain deviationinformation to a dynamic gain equalizer 110a which then adjusts theoptical signal 101 on a channel-by-channel basis to optimize the powerin each channel. Thus, any differences in gain accumulated through thetransmission of the optical signal 101 because of random gain or throughamplification by the In-line amplifiers 106 are corrected based upon theevaluation of the Optical spectrum analyzer 108a and implemented by theDynamic gain equalizer 110a.

The process repeats as the optical signal 101 is transmitted throughIn-line ampolfiers 106c, 106d, 106e, 106f and 106g. The optical signal101 is analyzed by the optical spectrum analyzers 108b and 108c once thesignal reaches In-line amplifiers 106d. The optical spectrum analyzer108b and 108c identify any gain differences on a channel-by-channelbasis of the optical signal 101 before transmitting these gaindifferences to the dynamic gain equalizers 110b and 110c. The dynamicgain equalizers 110b and 110c then correct for gain differencesaccumulated, regardless if the gain differences are caused by theIn-line amplifiers 106 or through polarization dependent loss.

The number of In-line amplifiers 106 is determined based upon thedistance the conventional optical network 100 must cover. Thus, whetherthe conventional optical network 100 is a long haul or ultra long hauloptical network, the frequency in quantity of In-line amplifiers 106must be determined. Further, the fiber characteristics, thecharacteristics of the amplifiers, and characteristics of the powernecessary for the optical signal 101 determine the quantity ofamplifiers 106 needed throughout the conventional optical network 100and also determine the quantity of analyzers 108 and equalizers 110necessary in the conventional optical network 100. Therefore, theoptical spectrum analyzer 108 and the dynamic gain equalizers 110 mayonly repeat every nth In-line amplifier 106 where n will be determinedbased upon the characteristics described above. In the example shown inFIG. 1, the Optical spectrum analyzers 108 and the dynamic gainequalizers 110 are provided at every two In-line amplifiers 106.However, the frequency could be increased or decreased based upon needof the system.

One constant of the conventional optical network 100 is that the Opticalspectrum analyzer 108 and the dynamic gain equalizer 110 are co-locatedat the same in-line amplifier 106. Therefore, the analysis andequalization of the optical signal 101 occurs at one specific point inthe optical network and is typically located at an in-line amplifier 106site.

Turning to FIG. 2, a graph, according to the prior art, of theaccumulated gain of an exemplary channel of the optical signal versus adistance is shown. The graph of FIG. 2 illustrates an exemplary channelof the optical signal 101; graphs of other channels of the opticalsignal 101 may form different slopes, however, the graph of accumulatedgain versus distance of all channels of the optical signal 101 will havea common element of reducing the accumulated gain to zero at a specificdistance. The graph of FIG. 2 demonstrates that at every distance x, theaccumulated gain of an exemplary channel of the optical signal 101 isreduced to zero. Therefore, referring to the embodiment shown in FIG. 1,the distance x would represent the distance between three in-lineamplifiers, for example In-line amplifiers 106b-106d. As can be seenfrom the graph of FIG. 2, the gain of one channel of the optical signal101 is linear over distance. Therefore, as is seen in FIG. 2, the gainof the exemplary channel of the optical signal 101 increases twodecibels through every span where the span equals the distance betweentwo adjacent in-line amplifiers 106. The linear line 200 represents thetotal gain of the exemplary channel of the optical signal 101 betweenequalization points, which is represented as distance x. Thus, the gainof the channel equals four decibels over this span. The distance x 204is the distance between two in-line amplifiers 106, for example, thedistance between In-line amplifier 106b and In-line amplifier 106d.

As the gain increases on any channel of the optical signal 101, theintegrity of the signal is reduced. Therefore, the greater the areadefined by the triangle formed by lines 200, 204 and 206 the greaterthan accumulated gain 202 and the greater the distortion of the opticalsignal 101. The graph of FIG. 2 is exemplary of the accumulated gaincaused by the effect of the In-line amplifiers 106 providing a constantpower boost across the multiple channels of the optical signal 101, andfor illustrative purposes does not represent any gain caused bypolarization dependent loss or any other random gain accumulation.However, one skilled in the art would recognize that the graphicalrepresentation of the accumulated gain versus the distance may be shownwith varying accumulated gain over specific distances or may be shownwith a non-linear curve representing the accumulated gain over aspecific distance.

Referring now to FIG. 3, a graphical representation of the amount ofloss attributed to wavelengths commonly implemented in opticaltransmission systems decibels (dBs) is shown. A curve 304 is shown inFIG. 3 representing the loss in dBs per kilometer versus the specificwavelengths of the channels of the optical signal transmitted over anoptical network. The curve 304 is non-linear and represents the varyingamount of loss experienced by specific channels of an optical signal asit is transmitted over the optical network. Thus, in the C-band range300, the amount of loss decreases generally as the wavelength isincreased from 1530 to 1560 nms. However, as the wavelength increasesfrom 1570 to 1610 nms to the L-band range 302, the loss begins togenerally increase. Therefore when a constant power boost is applied byan In-line amplifier to all channels of the optical signal, where eachchannel is comprised of a different wavelength, the amount of gain perchannel is not uniform and gain differences are propagated through theoptical network.

Referring now to FIG. 4, an exemplary embodiment of an optical networkaccording to the present invention is shown. Terminals 400 and 402 areprovided and connect the optical network 401 to local optical networks(not shown). An optical signal 403 is transmitted from the terminal 400along an optical line 410. For illustrative purposes only, the opticalnetwork 401 is shown in a unidirectional manner. However, both ofterminals 400 and 402 can act as transmission and/or receiving terminalsand it is expected that the optical network 401 can be implemented as abi-directional optical network allowing the transmission of opticalsignals from either terminal 400 or terminal 402.

After a predetermined distance, an in-line amplifier 404 is connected tothe optical line 410. The repeating predetermined distance of thein-line amplifier 404 is determined as discussed previously. In oneembodiment of the invention, erbium-doped fiber amplifiers areimplemented as the in-line amplifiers 404, however, a wide range ofamplifiers can be implemented without detracting from the spirit of theinvention. Once the optical signal 403 travels across this predetermineddistance, the In-line amplifier 404a boosts the power of each channel ofthe optical signal 403. In one embodiment, at start up of the opticalnetwork 401, the in-line amplifier 404 do not initially boost theoptical signal 403 as the optical signal 403 is transmitted along theoptical network 401 as no prior analysis of the optical signal 403 hasbeen completed. An optical spectrum analysis 406 located within theoptical system 401 analyzes the optical signal 403 to determine theamount of energy loss experienced by the optical signal 403 throughtransmission over the optical network 401. The optical spectrum analyzer406 then transmits this analysis data to the in-line amplifiers 440 andcommands the in-line amplifiers 404 to boost the channels of the opticalsignal 403 by an amount necessary to maintain a constant power level ofthe optical signal 403 across the optical network 401. This fine turningof the optical network 401 occurs when the optical network is initiallyinitiated, however, the optical spectrum analyzer 406 continuouslymonitors the strength of the optical signal 403 and can periodicallydirect a modification to the amounts of power boosted by a specificIn-line amplifier 404 during continuous use of the optical network 401to correct this gain tilt. In another exemplary embodiment the in-lineamplifiers' 404 output power is monitored by each in-line amplifier.This self-monitoring allows for the average power out of each in-lineamplifier 404 to remain approximately constant. As an additional step tothe self-monitoring, a measurement at the Optical spectrum analyzer 406determines the need for fine turning of the in-line amplifiers 404output power/average gain of the in-line amplifier 404.

When the optical signal 403 has its power boosted by the In-lineamplifier 404a, each channel of the optical signal 403 has the powerboosted with a consistent amount from the In-line amplifier 404a. In onedisclosed embodiment there are forty channels present in the opticalsignal 403. All forty channels of the optical signal 403 are boosted bythe same power level from the In-line amplifier 404a. As was disclosedwith FIG. 3, the power loss per channel varies according to thewavelength of that particular channel. Therefore, the per channel lossvaries and the gain difference is amplified and propagated by theIn-line amplifier 404a as it is transmitted from In-line amplifier 404ato In-line amplifier 404b. Once the optical signal is received by theIn-line amplifier 404b, the optical signal 403 is again boosted on achannel-by-channel basis by the In-line amplifier 404b. However, priorto the optical signal 403 being transmitted to the In-line amplifier404c, the Optical spectrum analyzer 406a analyzes the optical signal 403on a channel-by-channel basis to determine the accumulated gaindifference from the expected or optimal power level for each channel.

The optical spectrum analyzer 406 also determines the power boost tocorrect gain tilt propagated by the In-line amplifiers 404 by averagingthe power level of all channels and then applying a boost so that theaverage level of all channels equal the pre-determined power level ofthe optical signal 403 and further determines the accumulated gain orloss on a channel-by-channel basis of the power level as compared to thepre-determined optimal power level. The optical spectrum analyzer 406transmits the power boost data to the in-line amplifiers 404 over line414 via an optical supervising channel 412. Once the Optical spectrumanalyzer 406a determines the amount of gain differential for eachchannel of the optical signal 403, the optical signal analyzer 406atransmits these gain differentials to the dynamic gain equalizer 408over line 418 via an optical supervising channel 412. The opticalsupervising channel 412 is a management channel and supervisoryinformation is transmitted via this ethernet channel at 100 megabits to1 gigabit. The rates of transmission of information over the opticalsupervising channel 412 may vary without detracting from the spirit andscope of the invention. In another embodiment, the management orsupervisory channel can be implemented as a public telephone network orInternet line without detracting from the spirit of the invention. Thetransmission of management or supervisory information over themanagement or supervisory channel does not require the use of fiber andis not necessarily one of the wavelength channels of the optical line410. In one disclosed embodiment, the management and supervisory channelare implemented as the optical supervising channel 412 which transmitsmanagement supervisory information optically to the devices connectedwithin the optical network 401. The Optical spectrum analyzer 406atransmits the gain differential of each channel of the optical signal403 to the dynamic gain equalizer 408a which is co-located with in-lineamplifier 404a. The dynamic gain equalizer 408a receives the gaindifferentials transmitted by the Optical spectrum analyzer 406a throughthe optical supervising channel 412 over communication line 416. Thedynamic gain equalizer 408a and the Optical spectrum analyzer 406a arenot co-located with the same in-line amplifier 404. The dynamic gainequalizer 408a is located at a point between Optical spectrum analyzers406 or between the transmitting terminal 400 and the first Opticalspectrum analyzer 406a. Therefore, the dynamic gain equalizer 408acorrects the optical signal 403 at the first In-line amplifier 404abased upon the signal deviation present at the Optical spectrum analyzer406a located at In-line amplifier 404b. This methodology is repeated fordynamic gain equalizers 408b, 408c and optical spectrum analyzers 406band 406c. Thus, the optical signal 403 is modified at in-line amplifier404c by the dynamic gain equalizer 408b based upon informationtransmitted to the dynamic gain equalizer 408b by the optical spectrumanalyzer 408b based upon information transmitted to the dynamic gainequalizer 408b by the optical spectrum analyzer 406b located within-line amplifier 404d. The dynamic gain equalizer 408c modifies theoptical signal 403 at in-line amplifier 404e based upon data transmittedfrom the optical spectrum analyzer 408c located with in-line amplifier404f. Therefore, the dynamic gain equalizer 408 correct the opticalsignal 403 at one subset of in-line amplifier 404 based upon the signaldeviation analyzed at the optical spectrum analyzer 406 located at asecond subset of in-line amplifiers 404.

In one embodiment, the dynamic gain equalizer 408 and the Opticalspectrum analyzer 406 are spaced evenly throughout the optical network401. The dynamic gain equalizer 408 are placed approximately in thecenter between two adjacent. Optical spectrum analyzers 406. The dynamicgain equalizer 408 closed to the transmitting terminal 400 is placedapproximately in the center between the transmitting terminal 400 andthe first Optical spectrum analyzer 406a. A wide variety of alignmentschemes can be implemented without detracting from the spirit of theinvention as long as at least one dynamic gain equalizer is notco-located with one Optical spectrum analyzer. In one embodiment, thecontrol of the optical supervisory channel 412 is implemented throughthe use of a token which is transmitted to each device throughout theoptical network 401. For example, when the Optical spectrum analyzer406a has the token, that Optical spectrum analyzer 406a controls thegain tuning of the optical network 401 and can then transitamplification and gain differential information to the in-lineamplifiers 404a and 404b and the dynamic gain equalizers 408a. Once thisinformation has been transmitted and the in-line amplifiers 404a and404b and the Dynamic gain equalizer 408a adjust the power appropriately,the Optical spectrum analyzer 406a sends the token downstream of theoptical spectrum network 401, allowing another deice to transit theoptical supervisory channel 412. The Optical spectrum analyzers 406a, inone embodiment, are in communication with the in-line amplifiers 404 anddynamic gain equalizers 408 which are located prior to the Opticalspectrum analyzer 406. Thus, each Optical spectrum analyzer 406maintains communication with the dynamic gain equalizer 408 and thein-line amplifiers 404 that are immediately preceding it and are not incommunication with any other Optical spectrum analyzers 406. However,optional communication and control schemes are available and can beimplemented without departing from the spirit of the invention.

Referring now to FIGS. 4, 5, 6 and 7, the optical signal manipulationaccording to the present invention are shown. In FIG. 5, a graphicalrepresentation of a typical optical spectrum analysis conducted by theOptical spectrum analyzer 406 is shown. As expected, the gain differencevaries from channel to channel and extends to approximately 1.5 dB's forchannels 7 and 8 to 4.5 dB's for channel 26. Thus, depending upon thewavelength selected for each channel, the gain difference varies fromchannel to channel. The Optical spectrum analyzer 406 uses thisinformation to determine the average gain difference of the opticalsignal from the pre-determined optical level and uses this informationto adjust the power boost level of the In-line amplifiers 404 and thenthe Optical spectrum analyzer 406 determines on a channel-by-channelbasis the amount of gain (or loss) necessary for each channel so thateach channel's power equals the desired or optimal power level. Thisinformation is then transmitted from the Optical spectrum analyzer 406ato the Dynamic gain equalizer 408a through the optical supervisingchannel 412. The dynamic gain equalizer 408a the modifies the opticalsignal 403 at the In-line amplifier 404a on a channel-by-channel basisto compensate for the total amount of gain (or loss) that will beaccumulated once the signal reaches the Optical spectrum analyzer 406aat the In-line amplifier 404b. The dynamic gain equalizer 408a adjuststhe power associated with each channel of the optical signal 403 so theaccumulated gain is zero once the optical signal 403 reaches the In-lineamplifier 404b. To accomplish this task, the dynamic gain equalizer 408amust adjust the power associated with each channel of the optical signal403 below the accumulated gain of zero. This can be seen in FIG. 6 whichgraphically represents the accumulated gain versus distance over anexemplary single channel of the optical signal 403 as it is transmittedover the optical network 401. For the exemplary channel of the opticalsignal 403, the accumulated gain caused by the loss differencespropagated through the amplifiers 404 is shown as a linear gain overdistance. The amount of gain for an exemplary channel of the opticalsignal 403 increases 2 dB's from the terminal to the first in-lineamplifier 404a. The optical signal 403 on a channel-by-channel basis isthen equalized by the dynamic gain equalizer 408a based upon theinformation received from the optical spectrum analyzer 406a. Therefore,in this example, the accumulated gain according to the spectrum analyzer406a for this channel was a 4 dB accumulation. The dynamic gainequalizer 408a adjusts this channel with a 4 dB difference so thechannel possesses the proper power level when the signal reaches theOptical spectrum analyzer 406a. When the dynamic gain equalizer 408amodifies this channel of the optical signal 403 at the in-line amplifier404 location, the accumulated gain of this channel of the optical signal403 is a negative 2 dBs. This equalization is demonstrated by line 608.As the optical signal 403a is transmitted from the in-line amplifier404a to the in-line amplifier 404b, the accumulated gain continues toincrease a constant amount until it reaches point x 602 whichcorresponds with the in-line amplifier 404b. As expected, theaccumulated gain of this channel at the Optical spectrum analyzer 406a,which is co-located with the in-line amplifier 404b at a distance x 602,is zero. The dynamic gain equalizer 408a remembers, through the use of amemory mechanism located at the dynamic gain equalizer 408a, the amountof gain difference per channel that the dynamic gain equalizer 408c hasreceived from the Optical spectrum analyzer 406a. The Optical spectrumanalyzer 406a at distance x 602 again analyzes the optical signal 403 ona channel-by-channel basis to determine if the pre-equalized values doindeed correctly compensate the optical signal 403. If the accumulatedgain of any channel is not zero, then the Optical spectrum analyzer 406atransmits this information via the optical supervisory channel 412 tothe dynamic gain equalizer 408a to direct the dynamic gain equalizer408a to adjust the amount of gain equalization to those specificnon-zero channels.

It should be noted that during start-up procedures, the integrity of theoptical signal 403 is not maintained until a series of adjustments areperformed on the optical signal 430. Therefore, there will be a periodof time upon the start-up of the transmission of the optical network 401in which the data transmitted over the optical signal 403 will beinvalid. However, once the integrity of the optical signal 403 isestablished, then the process described above continues to modify theamount of gain equalization necessary over time.

A benefit of the present invention occurs because the pre-emphasis ofthe gain equalization ensures that the average power over the n spans,where n equals the number of in-line amplifiers 404 for each channel ofthe dense wavelength division multiplexed system, is the expected oroptimal power level. Further, the effects of random wavelength losses,as well as time varying polarization dependent losses are reduced by afactor of 2 by compared to the conventional approach. This improvementallows for twice as large component tolerances or with the samecomponent parameters induces only half the penalty on the system margin.This improvement is demonstrated by the area formed by trianglesoutlined by lines 600, 608 and the distance ½× which define area 606.Area 606 plus area 604 (a negative value) are approximately zero. Thesystem distortion from the fixed gain deviation has been nearlyeliminated. Thus, when compared to area 202 of the prior art, a largebenefit is realized. As the accumulated gain remain closer to zero (−2dBs to 2 dBs) without straying as far as −4 dBs to 4 dBs in theconventional systems, the overall integrity of the optical signal 403 isincreased.

FIG. 7 shows the gain difference power level in dB's of the channels ofthe optical signal 403 at the point of the optical spectral analyzer 406in the optical network 401. Thus, the gain differential in dB's perchannel of the optical signal 403 is constant is a constant zero. Byhaving an average power deviation of zero, as is obtained in theembodiment disclosed in FIG. 4 through FIG. 7, the distortions of theoptical signal pulse shape are greatly reduced.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof of various changes to the size,shape, materials, components and order may be made without departingfrom the spirit of the invention.

1. A power equalization system for use in an optical transmissionsystem, the system comprising: an optical line, wherein the optical lineincludes at least one transmission channel; a plurality of amplifierscoupled to the optical line, wherein the amplifiers are spacedperiodically throughout along the optical line; a plurality of Opticaloptical spectrum analyzers coupled to the optical line, wherein theOptical optical spectrum analyzers are spaced periodically throughoutalong the optical line and wherein the Optical optical spectrumanalyzers generate analysis data; and a plurality of equalizers coupledto the optical line, wherein the equalizers are spaced periodicallythroughout along the optical line and wherein the equalizers equalizethe power on the channels; a management line channel for transmittingmanagement data coupled to the pluralities of amplifiers, Opticaloptical spectrum analyzers and equalizers; wherein the plurality ofOptical optical spectrum analyzers are collocated co-located with aportion of the plurality of amplifiers and wherein the plurality ofequalizers are collocated co-located with a second portion of theplurality of amplifiers and wherein at least one of the plurality ofOptical optical spectrum analyzers is not collocated co-located with theplurality of equalizers; and wherebywherein analysis data generated bythe Opticaloptical spectrum analyzers is transmitted via the managementchannel to the equalizers for use by the equalizers in equalizing thepower of the channels of the optical line.
 2. The system of claim 1,wherein the amplifiers are spaced periodically along the optical lineand include in-line amplifiers.
 3. The system of claim 1, wherein theequalizers are spaced periodically along the optical line and areconfigured to equalize the power of each channel individually.
 4. Thesystem of claim 1, wherein the equalizers are spaced periodically alongthe optical line and include dynamic gain equalizers.
 5. The system ofclaim 1, wherein the analysis data transmitted by the Optical opticalspectrum analyzer is transmitted upstream.
 6. The system of claim 1,wherein the plurality of amplifiers include: a plurality of erbium-dopedfiber amplifiers coupled to the optical line and the management linechannel, wherein the erbium-doped fiber amplifiers adjust the power ofthe optical line to counteract gain tilt; wherebywherein theerbium-doped fiber amplifiers counteract gain tilt through theadjustment of the power of all channels collectively at the amplifiers.7. The system of claim 6, wherein the plurality of erbium-doped fiberamplifiers adjust the power of the optical line to counteract the gaintilt from Stimulated Raman Scattering.
 8. The system of claim 6, whereinthe plurality of erbium-doped fiber amplifiers adjust the power of theoptical line to counteract the gain tilt from non-uniform fiber loss. 9.The system of claim 6, wherein the plurality of erbium-doped fiberamplifiers adjust the power of the optical line to counter actcounteract the gain tilt based upon the analysis data transmitted viathe management line.
 10. The system of claim 1, wherein the plurality ofequalizers are spaced periodically along the optical line and areconfigured to equalize the power on the channels so the average powerover a periodic spacing is zero.
 11. The system of claim 1, wherein themanagement line channel includes an optical supervisory channel.
 12. Thesystem of claim 11, wherein the optical supervisory channel includes oneof the transmission channels of the optical line.
 13. The system ofclaim 1, wherein the management channel includes a public telephonenetwork.
 14. The system of claim 1, wherein the management channelincludes the Internet.
 15. A method of gain equalization of an opticaltransmission system, the method comprising the steps of: transmitting anoptical signal over at least one optical channel of the opticaltransmission system; transmitting a management signal over a managementline channel of the optical transmission system; amplifying the opticalsignal at predetermined amplifying positions in the optical transmissionsystem; analyzing the optical signal at a first portion of thepredetermined amplifying positions; determining an optical spectrum gainfrom the optical signal analysis; transmitting an optical spectrum gainsignal including the optical spectrum gain to an equalizer; andequalizing the optical signal based upon the received optical spectrumgain at a second portion of the predetermined amplifying positions;wherein at least one position of the first portion of predeterminedamplifying positions is not collocated co-located with a predeterminedamplifying position of the second portion.
 16. The method of claim 15,wherein the step of transmitting an optical signal over at least oneoptical channel includes transmitting optical signals over a pluralityof optical channels.
 17. The method of claim 16, wherein the steps ofanalyzing the optical signal and determining the optical spectrum gainincludes analyzing each optical signal channel and determining theoptical spectrum gain for each optical signal channel.
 18. The method ofclaim 17, wherein the step of equalizing the optical signal includesequalizing each channel of the optical signal based upon the opticalspectrum gain of each channel.
 19. The method of claim 15, wherein theoptical spectrum gain average over a predetermined span is zero.
 20. Themethod of claim 15, wherein the equalizing of the optical signalincludes dynamic gain equalization.
 21. The method of claim 15, whereinthe transmission of the optical spectrum gain signal is upstream. 22.The method of claim 15 further comprising: determining an amount of gaintilt in the optical signal based upon the optical signal analysis;transmitting a gain tilt signal including the amount of gain tilt of theoptical signal to an one or more erbium-doped fiber amplifiers;adjusting the power of the optical signal with the erbium-doped fiberamplifiers based upon the amount of gain tilt, wherein the erbium-dopedfiber amplifiers counteract gain tilt through the adjustment of thepower of all channels collectively at the amplifiers.
 23. An opticaltransmission system, comprising: a plurality of optical transmissionchannels coupled at a first end to a first terminal and at a second endto a second terminal; an optical spectrum analyzer coupled to theoptical transmission channels at a first intermediate location betweenthe first end and the second end; an equalizer coupled to the opticaltransmission channels at a second intermediate location between thefirst end and the second end, wherein the equalizer is spaced apart fromthe optical spectrum analyzer; wherein the system is configured totransmit analysis data generated by the optical spectrum analyzer to theequalizer for use by the equalizer in reducing power differences in theoptical transmission channels.
 24. An optical transmission system asrecited in claim 23, further comprising a management channel, whereinsaid management channel is configured to carry said analysis data fromthe optical spectrum analyzer to the equalizer.
 25. An opticaltransmission system as recited in claim 23, further comprising a firstamplifier co-located with the optical spectrum analyzer and a secondamplifier co-located with the equalizer, the first and second amplifiersbeing coupled to the optical transmission channels and configured toamplify optical signals in said channels.
 26. An optical transmissionsystem as recited in claim 23, further comprising a management channel,wherein said management channel is coupled to said first and secondamplifiers such that commands from said optical spectrum analyzer arecommunicable via said management channel to said first and secondamplifiers.
 27. The optical transmission system of claim 23, furthercomprising an amplification location, wherein the first and secondintermediate locations are not co-located with the amplificationlocation.
 28. The optical transmission system of claim 27, wherein theamplification location is between the first intermediate location andthe second intermediate location.
 29. A method for use in an opticaltransmission system, comprising: amplifying an optical signal at one ormore amplifying positions in the optical transmission system; analyzingthe optical signal at a first position and determining an opticalspectrum gain; transmitting an optical spectrum gain signal to anequalizer at a second position spaced apart from said first position;and reducing spectral component power differences in the optical signalat said first second position based upon the received optical spectrumgain.
 30. A method as recited in claim 29, wherein said spectrum gainsignal is transmitted over a management channel to said equalizer.
 31. Amethod as recited in claim 30, further comprising transmitting a commandsignal over the management channel to control one or more amplifiers.32. A method as recited in claim 29, wherein said second positioncorresponds to one of said amplifying positions.
 33. A method as recitedin claim 32, wherein the optical signal is amplified at first and secondamplifying positions, said first position corresponds to one of saidamplifying positions, and said second position corresponds to adifferent one of said amplifying positions.
 34. A system for use in anoptical transmission system, comprising: at least one amplifierconfigured to amplify an optical signal at one or more amplifyingpositions in the optical transmission system; at least one equalizer; ananalyzer configured to analyze the optical signal at a first position,to determine a spectrum gain, and to transmit a spectrum gain signal tosaid equalizer; wherein said equalizer is located at a second positionspaced apart from said first position, and is configured to reducechannel-to-channel power differences in the optical signal at said firstsecond position based upon the received spectrum gain.
 35. A system asrecited in claim 34, wherein said gain signal is transmittable over achannel to said equalizer.
 36. A system as recited in claim 35, whereinsaid at least one amplifier is configured to be controlled via a commandsignal received over said channel.
 37. A system as recited in claim 34,wherein said second position corresponds to one of said amplifyingpositions.
 38. A system as recited in claim 37, wherein the opticalsignal is amplifiable at first and second amplifying positions, saidfirst position corresponds to one of said amplifying positions, and saidsecond position corresponds to a different one of said amplifyingpositions.
 39. A system as recited in claim 34, comprising a pluralityof equalizers spaced periodically along an optical line of said system.40. A system as recited in claim 34, wherein said equalizer comprises adynamic gain equalizer.
 41. An optical transmission system, comprising:an optical channel; a management channel; at least one amplifierstationed at an amplifying position along said optical channel; at leastone equalizer stationed at an equalizing position along said opticalchannel; an amplifier stationed at an analyzing position along saidoptical channel and configured to generate a spectrum gain signal and totransmit said spectrum gain signal via said management channel to saidat least one equalizer.
 42. A system as recited in claim 41, furthercomprising at least one equalizer, wherein said equalizer is located atan equalizing position spaced apart from said analyzing position, and isconfigured to reduce channel-to-channel power difference in an opticalsignal at said equalizing position based upon the spectrum gain signal.43. A system as recited in claim 42, wherein said equalizing positioncorresponds top said amplifying position.
 44. A system as recited inclaim 43, wherein said equalizer comprises a dynamic gain equalizer. 45.A method for use in an optical transmission system, comprising:amplifying an optical signal at one or more amplifying positions in theoptical transmission system; analyzing the optical signal at a firstposition and determining an optical spectrum gain; transmitting anoptical spectrum gain signal via a management channel to a remotelocation.
 46. A method as recited in claim 45, wherein said spectrumgain signal is transmitted over said management channel to an equalizerat a second position spaced apart from said first position.
 47. A methodas recited in claim 46, further comprising reducing spectral componentpower differences in the optical signal said second position based uponthe optical spectrum gain.
 48. A method as recited in claim 25, furthercomprising transmitting a command signal over the management channel tocontrol one or more amplifiers.
 49. A method as recited in claim 46,wherein said second position corresponds to one of said amplifyingpositions.
 50. A method as recited in claim 46, wherein the opticalsignal is amplified at first and second amplifying positions, said firstposition corresponds to one of said amplifying positions, and saidsecond position corresponds to a different one of said amplifyingpositions.